Field of the Invention
The present invention relates to an apparatus and method for working electrically conductive materials by electric erosion, particularly but not exclusively to apparatus for applying a surface finish to a workpiece.
Related Art
The surface texture of some materials, for example sheet metal, can sometimes be of great importance. In the automobile industry, for example, it is important that sheet metal used for the bodywork of cars has a surface texture which produces good results under press and which gives a good appearance to the final paint finish.
Surface texture is usually applied to sheet metal by rolling with steel rolls, the surface texture of the roll being transferred to the sheet metal. The surface parameters of the rolls used are critical and must be closely controlled if the desired surface texture of the sheet metal is to be achieved.
It is known to texture the rolls by a process of blasting the roll surface with steel shot. The process is, however, difficult to control because of the wide range of variables involved, such as shot hardness, velocity and angle of impingement. In addition, it is now the trend to increase the life expectancy of the rolls by using very hard alloy combinations. Shot blast texturing of these hard rolls is very difficult.
It has previously been proposed to use the technology of electro-discharge machining (E.D.M.) to texture the surface of the rolls. This involves placing an electrode adjacent a roll to be textured and applying a pulse voltage across the gap, (usually though an electrolytic fluid) between the roll and electrode. A spark discharge will occur at the point where the electrode and roll are closest, the spark heating the surface of the roll and creating a minute crater therein. Repeated discharges at high frequency cause steady high-energy electrical discharges which produce many minute craters per second in the roll at various points on the roll surface, depending which point is closest to the electrode at the time, resulting in a uniform surface finish. E.D.M. is very controllable and is easily able to surface texture hard rolls.